The Optical Signal-to-Noise Ratio (OSNR) is a conventional measure of the quality of a signal carried by an optical telecommunication link. Under normal and proper operating conditions, the OSNR of an optical communication link is typically high, often in excess of 15 dB or 20 dB, or even greater. The dominant component of the noise in an optical communication link is typically unpolarized Amplified Spontaneous Emission (ASE) noise, which is a spectrally broadband noise source contributed by the optical amplifiers in the link.
Some methods exist for characterization of ASE noise on optical telecommunication signals based on an in-band analysis of the optical signal. Such methods include methods referred to as “polarization-nulling” methods (see J. H. Lee et al., “OSNR Monitoring Technique Using Polarization-Nulling Method”, IEEE Photonics Technology Letters, Vol. 13, No. 1, January 2001) as well as variants thereof, and the methods referred to as “differential polarization response” methods (see International Patent Application Publication WO 2008/122123 A1 to Gariépy et al.; and WO 2011/020195 A1 to He et al., both applications being commonly owned by the Applicant).
Polarization-nulling methods exploit the fact that the signal peak is generally polarized whereas ASE noise is generally unpolarized. By means of a polarization controller disposed before a linear polarizer, the combination serving as a polarization analyzer, it is possible to orthogonally align the polarization axis of the analyzer to the State Of Polarization (SOP) of the signal-under-test in order to find a condition where the signal peak is maximally suppressed. An optical spectrum trace is acquired while the signal peak is suppressed and reveals the in-band noise within the optical-channel bandwidth. In polarization-nulling methods, this in-band noise is normally assumed to only include ASE noise.
Differential polarization response methods involve the polarization-sensitive detection of an optical spectrum with Optical Spectrum Analyzer (OSA) means, where two or more optical spectrum traces are acquired under different polarization analysis conditions. However, unlike polarization-nulling methods, the differential polarization response approach does not require that the polarized signal be suppressed or close to the electronic noise floor of the measurement instrument for any of the acquired optical spectrum traces. Instead, differential polarization response methods employ a mathematical discrimination of the signal peak from the in-band noise in the acquired optical spectrum traces using calculations and a comparison between the acquired traces. This results in significantly less stringent requirements on both the polarization control of the signal-under-test and the quality of the OSA components (e.g. polarization extinction ratio), and leads to a significantly reduced measurement time in comparison to polarization-nulling methods. In differential polarization response methods, the in-band noise that is discriminated from the signal peak is also normally assumed to only include ASE noise.
However, in addition to ASE noise, there are other optical sources of distortion that may be present and significant on optical telecommunication signals for next generation high-speed networks. For example, the optical signal quality may be impaired by “linear crosstalk”, an interchannel phenomenon arising principally from adjacent channels. Upon application of in-band OSNR measurement methods of the art, such as prior-art polarization-nulling methods and prior-art differential polarization response methods, non-ASE optical noise such as linear crosstalk can be confused with ASE noise and/or distort the spectrum of the optical signal, thereby leading to inappropriate characterization of the in-band noise superposed on the optical signal-under-test.
Linear crosstalk within the channel bandwidth of a given channel-under-test arises from the presence of optical power that nominally is found entirely in an adjacent channel, propagating within the same optical fiber and characterized by a different central wavelength, but whose spectral extent is such that some of its optical power may fall within the channel-under-test and superpose upon in-band ASE noise. In the context of in-band OSNR measurements, linear crosstalk may affect the measurement uncertainty of in-band ASE noise measured by prior-art polarization-nulling methods and prior-art differential polarization response methods. Additionally, linear crosstalk itself also constitutes a non-ASE optical noise that may need to be quantified in order to more fully characterize the quality of an optical signal-under-test.
Accordingly, for advanced signal-quality characterization and OSNR measurement, there is a need for a method enabling characterization of linear crosstalk on optical signals and/or allowing improved characterization of ASE noise that discriminates between ASE noise and linear crosstalk on the signal-under-test.